1. Field of the Invention
The present invention relates to a method and an apparatus for controlling a servomotor and more particularly, to a method and an apparatus for controlling a servomotor in two inertial systems in which the servomotor and a load is mutually connected via an elastic connection element.
2. Description of the Related Art
In a flexible joint of a robot, a flexible arm, a universal structure and the like, a load and a servomotor for driving the load are connected via an elastic shaft, and form what is called two inertial systems. FIG. 1 schematically illustrates such two inertial systems. As shown in FIG. 1, a driving inertial system 3 at the side of a servomotor and a driven inertial system 5 at the side of a load are connected via an elastic connection element 7. In the drawing, symbols .omega., .theta., T.sub.e and J denote a velocity, a position, a torque command value and an inertial moment of the driving inertial system 3, respectively. Symbols .omega..sub.L, .theta..sub.L, T.sub.L and J.sub.L denote a velocity, a position, a disturbance torque and an inertial moment of the driven inertial system 5, respectively. A character K is a spring constant of the elastic connection element 7.
FIG. 2 is a block diagram showing a dynamic model of the two inertial systems of FIG. 1. The driving inertial system 3 outputs an output velocity a) according to a torque command value T.sub.e. Accordingly, the driven inertial system 5 produces a load velocity .omega..sub.L. Here, there are inevitably a difference (.omega.-.omega..sub.L) between the velocities and a difference (.theta.-.theta..sub.L) between the positions in the two inertial systems, the driving inertial system 3 and the driven inertial system 5 due to elasticity of the elastic connection element 7 via which the driving and driven inertial systems are connected each other. The differences between the velocities/positions become severe during abrupt acceleration and deceleration. The positional difference (.theta.-.sub.L) between the driving inertial system 3 and the driven inertial system 5 generates a resilient torsional torque, that is, T.sub.T =K(.theta.-.theta..sub.L)
As shown in FIG. 2, the resilient torsional torque T.sub.T acts on the driving inertial system 3 together with the torque command value T.sub.e, and thus affects the driving inertial system 3, that is, an output velocity of a servomotor. The resilient torsional torque T.sub.T also acts on the driven inertial system 5 together with the disturbance torque T.sub.L, and thus affects the driven inertial system 5, that is, a velocity of a load. Such an effect of the resilient torsional torque T.sub.T does not only lower stability in controlling of the system. but also generates a torsional vibration, to thereby further deteriorate stability of the system.
FIG. 3 is a block diagram showing an example of a conventional control apparatus for the two inertial systems. As shown in FIG. 3, the conventional control apparatus includes a PI (proportion and integration) controller 9 which produces a torque command value T.sub.e, for the driving inertial system 3, that is the servomotor. An output velocity .omega. of the driving inertial system 3 is feedback to a subtracter 1 provided in the preceding end of the PI controller 9. The subtracter 9 outputs a target velocity of the driven inertial system a, that is, an error value between a load velocity command value .omega.* and the output velocity .omega. of the driving inertial system 3, to the PI controller 9. The PI controller 9 operates to produce a torque command value T.sub.e for having an error between the load velocity command value .omega.* and the motor output velocity .omega. be access to zero as closely as possible. Since the motor output velocity .omega. reflects the effect of the resilient torsional torque T.sub.T by the elastic connection element 7, the PI controller 9 can reduce the effect of the resilient torsional torque T.sub.T during normal operation of the servo system.
However, during abrupt acceleration and deceleration, the resilient torsional torque T.sub.T acts on the driving inertial system 3 and the driven inertial system 5 alternatively, that is, in the form of vibration, and a amount of the position/velocity differences between the driving inertial system 3 and the driven inertial system 5 occurs, the PI controller 9 is limited to appropriately cope with the above occurrence. Thus, the occurrence of the vibration and the lowering of the system stability cannot be effectively prevented by using only the PI controller 9 in the two inertial systems.